Believe it or not, the chance that you will be cremated by an asteroid are three times greater than that of being killed by a bus or train. At least that's the conclusion of a book titled "Life: The Odds" by Gregory Baer .
If you are talking about small asteroids (less than, say, the size of a football field), the odds are long. Even though there are plenty of small asteroids floating around Earth's orbit path, they are too small to survive our atmosphere.
Medium-sized asteroids (more than 100 meters but less than one kilometer in diameter) could do devastating damage for hundreds of miles, but would not threaten global civilization. And the odds that one of these will hit Earth in the next 100 years are about 5 to 1. (5 to 1!)
And now for the big one . . .
If an asteroid two kilometers in diameter were to strike Earth, the impact would be like that of 5 million Hiroshima-sized nuclear bombs, extinguishing most living things. Many scientists now believe this is what happened 65 million years ago in Mexico's Yucatan Peninsula, eradicating the dinosaurs. According to NASA, an asteroid of this size collides with Earth every once or twice per million years.
Want to know more? NASA operates a website devoted entirely to this subject. Go to NASA's web site.
NASA